The present invention relates to lithographic patterning of a resist layer on a curved substrate, in particular with spherical concave curvature. More specifically, the invention relates to a lithographic method for producing an exposure pattern on a curved substrate field of a substrate, the substrate field comprising material sensitive to exposure to an energetic radiation. In a pattern transfer system a wide, substantially parallel beam of the energetic radiation is produced, and by means of the collimated beam a planar mask having a structure pattern, namely, a set of transparent windows to form a structured beam, is illuminated and the structure pattern is imaged onto the substrate by means of the structured beam, the substrate being positioned after said mask as seen in the optical path of the beam, producing a pattern image, namely, a spatial distribution of irradiation over the substrate.
In manufacturing semiconductor devices, one important step for structuring the semiconductor substrates is lithography. The substrate, for instance a silicon wafer, is coated with a thin layer of photosensitive material, called photo-resist. By means of a lithographic imaging system, a pattern is imaged onto the photo-resist, and the subsequent development step removes from the substrate either the exposed or the unexposed portion of the photo-resist. Then, the substrate is subjected to a process step such as etching, deposition, oxidation, doping or the like, the photo-resist pattern on the substrate covering those portions of the surface that shall remain unprocessed. The photo-resist is stripped, leaving the substrate with the new structure. By repeating this sequence, multiple structure layers can be introduced to form the semiconductor micro-circuits.
There has been a growing interest in patterning of curved surfaces at sub-micron dimensions, in particular with optical sensor arrays on concave surfaces, so-called focal surface arrays (FSA). Applications lie in the field of imaging technology, such as infra-red cameras and wide-field optical sensors. To date, the ability to reduce the size, or even miniaturize, infra-red cameras is limited by the size and weight of the optical components; these could be reduced by an order of magnitude if a spherical imaging array is used instead of flat one. Moreover, spherical imaging arrays enable simple, compact optical designs with ultra-wide fields-of-view. In comparison with flat designs, however, the patterning of curved surfaces on the one-micron scale is a challenge because of the large depth of field that is needed for the topographical variation of the substrate.
The inventors, in J. Vac. Sci. Technol B 17(6) pp. 2965-2969, 1999, have shown that shadow printing lithography, in particular ion beam proximity (IBP) lithography, has the resolution and depth-of-field required for the task of patterning FSAs. Lithographic printing methods as well as lithographic devices using electron or ion beams are discussed, for instance, by H. Koops in xe2x80x98Electron beam projection techniquesxe2x80x99, Chapter 3 of xe2x80x98Fine Line Lithographyxe2x80x99, Ed. R. Newman, North-Holland, 1980, pp. 264-282. Electrons and in particular ions have the advantage of very low particle wavelengthsxe2x80x94far below the nanometer rangexe2x80x94which allow of very good imaging properties, as e.g. discussed by Rainer Kaesmaier and Hans Lxc3x6schner in xe2x80x98Overview of the Ion Projection Lithography European MEDEA and International Programxe2x80x99, Proceedings SPIE, Vol. 3997, Emerging Lithography Technologies IV, 2000. Proximity printing using stencil masks is, however, not restricted to particle beam systems, but also possible with lithography systems based on photons, like EUV (Extreme UV) or X-ray lithography.
In shadow printing lithography (and likewise in projection lithography), the pattern to be imaged onto the photoresist-covered substrate is produced by using a mask or reticle having the desired pattern. For particle lithography systems, stencil masks are used in which the patterns to be projected are formed as apertures of appropriate shape in a thin membrane, i.e., a few micrometers thick. The mask pattern is built up from a number of apertures in a thin membrane through which the particle beam is transmitted to expose the resist-coated wafer in those areas required for device fabrication.
Lithographic patterning of curved substrates suffers from a complex of problems arising from the projection of the mask pattern onto substrate areas inclined with respect to the mask, which causes not only a distortion of the mask pattern as compared to the original mask pattern, but also a reduced local exposure dose density. Blur in shadow printing, such as IBP lithography, is due to imperfect collimation of the ions and so depends on the gap between the mask and the substrate surface. Thus, in the particular case of a concave substrate field, the blur, linewidth and exposure latitude all depend upon the radial distance from the center of the substrate field.
It is an aim of the present invention to overcome the above-mentioned problems and, in particular, to show a way to correct for the distortions incurred from the projection of the mask pattern onto the curved substrate field while avoiding other problems as, e.g., deterioration of pattern reproduction due to non-uniformity of dose density.
The invention provides a solution to the above task by a method as mentioned in the beginning wherein the center of curvature of the substrate field is positioned on a line as defined by a ray of the beam running through a pattern center defined on the mask within the area of the structure pattern, the windows of the structure pattern being arranged in a manner that along each radius from the pattern center, the radial spacing of said windows decreases with increasing radius from the pattern center, and wherein the direction of incidence of said beam onto the mask is varied through a sequence of inclinations with respect to the normal axis to the mask, the sequence of inclinations being adapted to merge those exposure pattern components which result from neighboring windows of the structure pattern, the exposure with respect to the sequence of inclinations superposing into a spatial distribution of exposure dose on the substrate, said distribution exceeding the specific minimum exposure dose of said resist material within only one or more regions of the substrate field, said region(s) forming the exposure pattern.
By virtue of the invention the design of self-supporting masks is possible which allow of a patterning of curved, in particular concave spherical, substrates with a flat mask. The decreasing spacing of the windows with increasing radial distance from the pattern center can suitably be employed to compensate for the effect of distortion due to the curvature of the substrate field.
Devices which are in particular suitable for the above method according to the invention are the lithography apparatus according to claim 14 and the lithography mask according to claim 15. In a preferred embodiment of the invention, the radial spacing of the windows follows the projected distances of uniformly-spaced points on the substrate field projected onto the mask plane. Thus, the imaging distortions upon projection onto the curved substrate are taken into account, simplifying the design of the respective mask pattern. In particular, the structure pattern can be a subset of an array of windows, the position of the windows determined by a two-dimensional array obtained from a regular two-dimensional array of uniformly-spaced points deformed by a transformation corresponding to a projection from the substrate field onto the mask plane. Preferably, the inclination range corresponds to the inclination range used to image an array of windows positioned on said regular two-dimensional array into a full-field exposure pattern on a planar substrate positioned at a distance equal to the radius of curvature of the substrate field.
In a further preferred embodiment, the substrate field has concave curvature and the center of curvature of the substrate field is positioned coinciding with the pattern center. This special geometry greatly enhances the imaging properties of the projection.
Moreover, it is advantageous if the windows of the structure pattern have uniform area. This ensures that every portion of the substrate is sufficiently exposed to radiation even if the substrate is locally inclined with respect to the incident beam.
More relaxed is the condition that the substrate pattern is composed of windows of varying shapes, the dimensions of the windows varying with increasing radius (i.e., distance from the pattern center) according to a contraction factor which is equal to one at the pattern center and decreases with increasing radius, wherein
(i) the radial window dimensions decrease according to said contraction factor, and
(ii) the window dimensions perpendicular to the respective radial directions increase according to the inverse of said contraction factor.
Here and in the following, the term xe2x80x98radiusxe2x80x99 (or xe2x80x98radialxe2x80x99) always refers to the half line (or the direction) from the pattern center to the respective point or window, as the case may be.
In another suitable variant, the substrate pattern is composed of windows of varying shapes, the shape of each window being derived from an original shape uniform to all windows by means of a deformation defined in terms of a contraction factor equal to the quotient of
(i) the length of a radial (i.e. oriented parallel to a radius from the pattern center) line element at the position of the respective window and
(ii) the length of the projection of said radial line element onto the substrate field,
wherein said original shape is shrunk along the radial direction by said contraction factor and stretched along the respective perpendicular direction by the inverse of said contraction factor. Preferably, this original shape is a square.
Advantageously the energetic radiation comprises electrically charged particles and the pattern transfer system is a particle optical imaging system. In particular, the energetic radiation may comprise ions, such as hydrogen or helium ions, and the pattern transfer system is an ion optical imaging system. In this case, it is further suitable if the direction of the beam is inclined by an electrostatic deflection means of the particle optical imaging system.
The invention can be used with a variety of applications, one of which is the patterning of resist layers by energetic radiation. In this case, the substrate field comprises a layer of resist material sensitive to exposure to an energetic radiation. For instance when using ion-beam radiation, there is a host of applications for spatially varying exposure to the ion radiation, such as converting a GaAs substrate into an insulating state by bombardement with, e.g., hydrogen or oxygen ions, doping of semiconductor materials, hardening of material against etching or abrasive attack, or influencing the fraction index by irradiation.